Dams may not be producing as much greenhouse gas emissions as thought, new research argues. It suggests that dammed reservoirs may contribute only 18.6% of the global emissions projected by other approaches. The findings come from incorporating relevant but often overlooked model data: baseline emissions from the same reservoirs if the dams didn't exist. Hydroelectricity has grown in popularity, and so too has the number of dams constructed around the world. These structures trap organic carbon, emitting methane and carbon dioxide. "High carbon emissions from reservoirs have sparked controversy over dams and even raised questions about the green credentials of hydropower," Wenqing Shi and colleagues write. They simulated dam and non-dam emissions from the same 5,346 dammed reservoirs globally, revealing that estimates of dams' greenhouse gas contributions are too large. Shi et al. found that this inaccuracy exists because conventional techniques haven't considered baseline emissions from dammed reservoirs under no-dam conditions. When incorporating this baseline, the authors established that damming leads to an average increase in carbon emissions of 21.5% – which is only 18.6% of previously estimated emissions. Shi et al. note this work does not consider downstream estuarine carbon emissions. "Future dam planning and environmental impact assessments should evaluate carbon consequences at the scale of the integrated river-estuary continuum," the team writes. "This holistic perspective will facilitate the development of low-carbon hydropower systems and provide a scientific basis for balancing renewable energy expansion with climate mitigation goals."
For reporters interested in how dams impact the environment, a July 2026 Science Advances study quantified how large dams in the U.S. cause widespread warming of rivers downstream, with potential consequences for fish spawning, harmful algal blooms, and carbon emissions, among other ecological impacts: www.science.org/doi/10.1126/sciadv.aeb5193
